A great deal of work has been done with the goal of developing self-cleaning coatings for windows and other substrates. One area of research has focused on photocatalytic coatings. Research in this area is founded on the ability of photocatalytic coatings to break down organic materials that come into contact with the coatings. One such photocatalyst appears to be titanium oxides (titanium dioxide, for example).
Windows may derive great benefit from photocatalytic coatings. For example, such windows may have self-cleaning characteristics. To the extent organic matter is deposited on such a window, the photoactive coating may act to oxidize the organic deposits, thereby having a cleaning effect on the surface of the window. To the extent residue survives this photocatalysis, such residue may be more easily removed by washing or, for outdoor window surfaces, by run-off rainwater.
Photocatalytic coatings have been developed which typically involve a titanium dioxide layer on a glass pane. The coatings are commonly provided with a relatively thick layer of titanium dioxide and/or a specific under-layer system designed for achieving high levels of photoactivity. Such photocatalytic coatings may be useful for absorbing ultraviolet radiation and photocatalytically degrading organic materials that may have collected on the coating. Thick titanium dioxide layers, unfortunately, produce high levels of visible reflectance, thus creating a somewhat mirror-like appearance. This high visible reflectance tends to exaggerate the appearance of dirt on a window.
Glass surfaces, such as windows, may have a photocatalytic (PCAT) coating applied in which the coating typically comprises a layer of titanium dioxide (TiO2) that may be roughly 250 to 300 angstroms (Å) thick. PCAT coatings in this thickness range may be visible to the unaided human eye, and their presence may therefore be relatively easy to detect. Furthermore, at coating thicknesses in this range, visible light reflectance is significantly higher than it would be for the same substrate without the PCAT coating (typically in the range of 7 to 10% higher). Devices have been developed that can help distinguish between PCAT coated surfaces at these coating thicknesses and non-coated surfaces.
More recently, PCAT coatings have been developed in which the coating is significantly thinner than that described above. For example, work has been done to reduce the thickness of the TiO2 layer in certain PCAT coated products to reduce and possibly eliminate reflectance and appearance differences between coated and non-coated surfaces, while maintaining the photocatalytic functionality of the coating.
The existence of thinner PCAT coatings may make it more difficult to distinguish between substrates coated with a “thin” PCAT coating and uncoated substrates. Visible differences between “thin” PCAT coatings and uncoated surfaces may be negligible. Furthermore, existing devices, which may have been designed for use with “thick” PCAT coatings (i.e., 250 to 300 Å), may not be able to distinguish surfaces with “thin” PCAT coatings from uncoated surfaces. These difficulties may pose additional challenges in the areas of manufacturing, quality assurance and distribution. For example, a customer or distributor receiving a shipment of the thin PCAT coated product may not be able to easily verify that they received the correct product due to the reduction in appearance and reflectance differences. New methods and devices for identifying the presence of “thin” PCAT coated products are therefore necessary.